Derivation of a weighted-average reflection coefficient for mesophyll cell membranes of Kalanchoë daigremontiana

In a study with the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perr. using the pressure probe, Rygol et al. (1987, Planta 172, 487-4493) calculated a value for the reflection coefficient (σ) for malate of 0.6. This value was derived from the relationship between measured changes in cell turgor pressure and malic-acid concentration, and would imply that malate was a relatively ineffective osmoticum. Here we show that the calculation of Rygol et al. (1987) involved the implicit assumption that xylem tension was constant with changing cell turgor pressure and osmotic pressure. This has been shown not to be the case using the pressure-chamber technique. We present an alternative method of deriving a weighted-mean value of a for K. daigremontiana and show that it is not significantly different from 1.0. © 1994 Springer-Verlag.